1. Field of the Invention
The present invention relates to an improvement in a semiconductor laser. Especially the present invention relates to a semiconductor laser made by a liquid phase epitaxial growth method.
2. Prior Art
Together with remarkable progress of light communication technology and disk type video recording technology, semiconductor lasers are regarded as most important devices to serve as light sources to be used in such technologies. Accordingly, there are great requirements on reliability and performance of such semiconductor lasers. That is, lasers of longer lifetime and lasing with more stable fundamental mode are required.
Various types of lasers have been hitherto developed, and a terraced substrate structure type has been also proposed in the U.S. patent application Ser. No. 40,182, now U.S. Pat. No. 4,296,387. FIG. 1 shows the structure of the terraced substrate structure type laser which comprises the following parts:
a substrate 1 of: n.sup.+ -GaAs, PA1 a first clad layer 3 of: n-Ga.sub.1-x Al.sub.x As, PA1 an active layer 4 of: non-doped Ga.sub.1-y Al.sub.y As, PA1 a second clad layer 5 of: p-Ga.sub.1-z Al.sub.z As, PA1 a contacting layer 6 of: p-GaAs, and PA1 a hetero-isolation layer 7 of: n-Ga.sub.0.5 Al.sub.0.5 As.
The layers 3, 4, 5, 6 and 7 are formed by sequential epitaxial growths on the substrate 1, which is formed in a terrace shape having a step part 101. Therein the first clad layer 3 has a triangular part 301 at the step part 101, and accordingly the active layer 4 is shaped on the triangular part 301, so as to have an oblique lasing region 8 defined between a higher bending portion 81 and a lower bending portion 82. In this structure, the thickness of the first clad layer 3 under the lasing region 8 is thicker than those under other parts of the active layer 4, and hence beam divergence measured in a plane perpendicular to a junction between the active layer 4 and the second clad layer 5 is small. That is, the triangular part of the first clad layer 3 at the thickest part is about 1 .mu.m thick, and a lower horizontal part and an upper horizontal part of the first clad layer 3 is 0.3 .mu.m thick. Therefore, the emission in the active layer 4 on the upper or lower horizontal part is absorbed by the substrate 1, while the emission in the lasing region 8 on 1 .mu.m thick triangular region 301 is not absorbed by the substrate 1 and makes a single spot lasing. Furthermore, the lasing region 8 is about 20% thicker than the horizontal parts, forming as a result a folded ridge type light waveguide, wherein effective refractive index is higher in the bent parts, resulting in further stability of lasing mode.
However, the laser of FIG. 1 has a problem that the lasing region 8 is disposed oblique to the horizontal upper face and lower face which are of (100) plane of the substrate 1, and accordingly the crystal of the lasing region is likely to be imperfect, thereby reducing lasing yield. Furthermore, the lower bending portion 82 is liable to obscurity, which leads to insufficiency of lasing light and resultant poor convergency of light in a plane perpendicular to the junction plane. Therefore, lasing in a fundamental mode of circular beam divergence is difficult.